1. Field of the Invention
This invention relates to a screw type connection for securely mounting a piston rigidly on a rod for use as one part of a piston and cylinder type linear hydraulic motor or actuator, for example.
2. Description of the Prior Art
Various types of connections have been used for mounting a piston on a rod for use as a part of a linear actuator, the other principal part being a cylinder. One known type is the screw type in which the rod contains external screw threads and the piston is mounted on the rod by means of mating internal screw threads. It is known to apply a torque load on such a connection, therewith tightening the mating threads sufficiently to apply an initial axial preload between the shoulder of the rod and piston, greater than the separating or the shoulder surface crushing force caused by hydraulic fluid pressure. Such an arrangement is not satisfactory in many severe applications, mainly because such an arrangement may accomplish tight contact only between a few thread flanks, instead of between all available thread flanks, due to manufacturing tolerances. Then, either a hydraulic transient shock load and/or a mechanical shock load, such as caused by bottoming out of the piston in extending mode may cause these few mating theads to yield whereby the initial preload would be immediately reduced to zero.
In either event, or both, the screw connection between the piston and the rod then will not be tight and this makes possible another phenomenon in a severe application. Non-uniform axial loading, plus cumulative tolerance may lead to bending of the cylinder and, albeit very small, relative rotation between the cylinder and the rod. With the piston tight against the cylinder wall this effects a small increment ratchet action on the piston and threads, which conventional normal restraints such as cotter pins, set screws and the like fail to control adequately. Consequently, after a sufficient amount of cycles of operation the piston may come off the rod with all the inherent damage.
The object of the present invention is to provide a screw type rigid connection for mounting a piston on a rod which is superior to prior piston and rod connections. A piston and rod connection which is similar in some respects to the present connection is shown in the U.S. Pat. No. 2,758,897 Naab. However, the present construction provides a piston and rod connection which can allow a shorter piston, requires less parts and is easier and less costly to manufacture than the construction of the said patent.